Typical diaphragm-type transducers and actuators employ a thin diaphragm of a circular, square or rectangular plan configuration. When this diaphragm is subjected to a force, the deflection of the diaphragm reflects the magnitude of this force. It is well known that the deflection of such a diaphragm is linear with applied force or pressure so long as the deflection is a small fraction of the thickness of the diaphragm. As the force or pressure is increased beyond this point, the deflection becomes a non-linear function of the applied force or pressure due to the stretching of the diaphragm. In many applications this non-linear travel limits the useful range of the device. Flat deflecting beams that stretch when subjected to an applied force or pressure have a similar problem. These diaphragms or beams are typically used as the movable element in pressure transducers and actuators, but can also be used in accelerometers, force transducers, or displacement transducers.
As diaphragms are made thinner, the above-mentioned non-linear deflection characteristics of the diaphragm is exacerbated. Hence, typically, to provide a low pressure transducer or actuator having a satisfactory linear operating range requires that the diaphragm be larger. This is unacceptable for many applications where there are size constraints. For example, it is important that pressure transducers formed using semiconductor materials be as small as possible.
Another problem arises when a flat diaphragm which is clamped at its edges, is subjected to a differential pressure. The central region of the diaphragm is bent into a curved shape rather than moving up and down in a piston-like manner. If, as typically is the case, the measurement of the deflection of the diaphragm is done by capacitor plates attached with one plate on the diaphragm and a second plate on a surface opposite the diaphragm, the measurement will be complicated because the shape of the diaphragm plate changes with applied force or pressure. Further, it is known that a relatively large movement of the diaphragm is advantageous when measuring the deflection by capacitive means since the change in capacitance is related to the reciprocal of the gap between the plates of the capacitor. For very sensitive flat diaphragms, the linear deflection is only a small fraction of the thickness of the diaphragm. This requires that the capacitor plates be positioned with a gap having a width which is also a fraction of the diaphragm thickness. Achieving such a small capacitor gap can greatly complicate the assembly of such a structure. If the small gap cannot be realized, the change in capacitance for a given applied force or pressure will be severely limited.
It is known that corrugated diaphragms provide certain advantages over flat diaphragms when utilized in a pressure transducer. For purposes of this specification, what is meant by a corrugation is a structure having one or more grooves separated by a thin section that allows compliant movement. For corrugated diaphragms, the compliance in the grooves substitutes for the stretching that would occur in flat diaphragms. The primary advantage of a corrugated diaphragm is that there is a more linear vertical travel per unit of applied force. For example, it is taught in Flat and Corrugated Diaphragm Design Handbook by Marco Di Giovanni (published by Marcel Dekker, Inc.) that a diaphragm of similar sensitivity and diameter will have more linear range and stiffness if it has a corrugated support rather than a flat support. Hence, corrugated diaphragms are utilized advantageously to increase the range of linear travel of the transducer as a function of applied force.
Therefore, in order to alleviate diaphragm non-linearity due to stretching, metallic structures have been used wherein the diaphragm is formed with corrugations. Corrugated diaphragms of this type have been found to exhibit a larger range of linear deflection to applied pressure, thereby minimizing the stretching effect. Typically, such diaphragms are used in conjunction with a push rod and beam to form a relatively complex pressure responsive mechanism. A problem with such diaphragms is that they must be machined or formed from suitable metals and are difficult to manufacture. Furthermore, the strain sensors located thereon need to be separately positioned and mounted on these corrugated structures, resulting in additional problems which affect performance within the linear range.
The above-described manufacturing and performance problems of corrugated diaphragm sensors are minimized if semiconductor processing techniques can be used. U.S. Pat. No. 4,467,656 to Mallon, et al. teaches that a convoluted diaphragm can be formed in silicon. Piezoresistive devices are diffused into the convolutions using integrated circuit methods. The result is a pressure transducer that can be fabricated from a silicon substrate by etching concentric recesses or corrugations on both sides of the substrate. The corrugations are surrounded by a rigid peripheral area.
U.S. Pat. No. 4,236,137 to Kurtz et al. discloses a pressure transducer having a semiconductor diaphragm with a central boss area of trapezoidal cross-section surrounded by a continuous groove. A plurality of piezoresistive sensors are formed on the diaphragm with a first sensor adjacent to the outer edge of the groove and a second sensor parallel to the first sensor and being adjacent to the inner edge of the groove. The groove is operative as a stress concentrating area for the sensors. It is known, however, that a single groove does not substantially improve the linearity of diaphragm travel over that of a flat diaphragm structure. Therefore, although this structure is useful in some applications for edge stress measurement, it would not be effective in many applications for the same reasons that flat diaphragms are not effective.
In many transducer applications, such as in accelerometers, it is necessary to measure the force perpendicular to the plane of the diaphragm. The accelerometer typically has a centrally positioned deflecting member that deflects in response to an applied force, the vertical travel of the deflecting member being a measure of the applied force.
A key problem with previous transducers utilizing corrugations is that there are stress points located within the corrugations that can adversely affect the operation of the transducer. It is known that corrugations formed by anisotropic etch techniques are trapezoidal in shape and that the trapezoidal corrugations will have stress concentrated in the corners of the corrugations. Hence, if an excessive amount of pressure is applied to the transducer, the corrugations may crack at those corners, rendering the transducer inoperative.
Mallon, et al., notes that isotropic etching could be used to provide a rounded configuration, but does not disclose how to achieve such a structure. In fact, Mallon, et al. teaches that the anisotropic etch is preferred and the stress problem caused by these types of corrugations is not addressed. It is well known that conventional isotropic etch techniques are difficult to control and the corrugations produced utilizing isotropic etch techniques may not be uniform, thereby causing stress to be still concentrated therewithin.
It is also known that producing corrugations utilizing known processing methods can be very difficult when thin diaphragms are formed for low pressure measurements. In a typical process, each side of a silicon material is masked utilizing typical photolithography techniques and then each side is etched into the desired pattern. This process typically requires precision alignment instruments to ensure that front and back surfaces match. If the surfaces do not match, then the resultant corrugations will not be properly formed, which seriously affects the performance of the transducers. In particular, when thin structures are formed (on the order of 0.5 .mu.m to 10 .mu.m), then misalignment becomes very significant, often to the point of rendering the transducer inoperative.
A final related problem with corrugated diaphragm structures is that the depth of the corrugations and the thickness of the deflecting member each affect the deflection characteristics of the structure. When the corrugated diaphragm structure is produced via the above procedure, then the deflecting member must be masked with the corrugations. In the resultant structure the corrugations can only be the same thickness as the deflecting member. Therefore, since the corrugation and deflecting member thicknesses are related, the dimensions of the transducer formed by these processes are limited by that dependence.
Accordingly, it is a principal object of the present invention to provide a semiconductor transducer or a semiconductor actuator that has increased linearity of travel per unit of applied force or pressure.
It is another object of the present invention to provide a method for producing a corrugated structure to be utilized with a transducer or actuator that overcomes some of the fabrication problems associated with known semiconductor processing techniques.
It is yet another object of the present invention to provide a semiconductor transducer or a semiconductor actuator which overcomes some of the problems associated with previous diaphragm transducer or actuator assemblies.
It is a further object of the present invention to enable corrugation formation to be less dependent on front-to-back alignment of the semiconductor starting material.
It is still a further object of the present invention to provide a transducer or an actuator which maximizes the deflection for a given applied force or pressure.
It is yet a further object of the present invention to provide an improved method for forming a semiconductor transducer or a semiconductor actuator which allows the predetermined thickness of the deflecting member to be independent of the predetermined thickness of the corrugations.